This invention relates to superconducting films and devices incorporating the films and a method of synthesizing the films and devices. The superconducting films exhibit a type of Josephson tunnel junction effect. The films can be microscopically inhomogeneous, for example, films including granular or columnar microstructures and can be some types of multilayer films which have inhomogeneities therein and combinations thereof. These films provide an AC to DC conversion effect in the absence and in the presence of an applied magnetic field. The films also can be multilayer films which do not have substantial inhomogeneities which provide an AC to DC conversion effect when a magnetic field is applied to the films. The microscopically inhomogeneous films are essentially compositionally homogeneous, but are structurally inhomogeneous. Both the multilayer films with inhomogeneities and the multilayer films without inhomogeneities therein are essentially compositionally homogeneous within each layer.
A Josephson tunnel junction generally includes a thin tunnel barrier layer sandwiched between two thin superconducting metallic layers. The voltage across a Josephson junction is capable of rapidly switching from zero volts to a finite voltage when a critical current is passed through the junction. The Josephson junction devices can be utilized in numerous types of electronic circuit elements, such as logic and memory circuits and devices including switches, diodes, transistors, amplifiers, rf and microwave detectors, generators and mixers, etc.
Conventional Josephson junctions generally are formed by thermally evaporating lead or other superconducting metal layer or film onto a substrate in an evacuated deposition chamber. The tunnel barrier layer can be formed by oxidizing the exposed surface of the lead layer. A second lead layer is then thermally evaporated onto the barrier layer. Other techniques have also been utilized to form the junctions, including the reactive ion beam-sputter deposition of a superconducting metal in the presence of oxygen.
In these prior devices, the superconducting layer deposited on the substrate generally is designated the base electrode and the second superconducting layer is designated the counter electrode. The voltage which is developed across a Josephson junction is determined by the materials from which the base and counter electrodes are formed. In the case of the lead-lead oxide-lead junctions, the voltage developed is approximately equal to twice the energy gap of lead or about 2.5 millivolts. Thus, to utilize these junctions in a high voltage switch, it is necessary to connect a plurality of the junctions in series. This typically is accomplished by numerous deposition steps or by a complicated masking procedure.
Numerous attempts to construct both natural and new crystalline analogue materials have been made with the aim of extending the range of material properties heretofore limited by the availability of natural crystalline materials. One such attempt is layering or compositional modulation by molecular beam epitaxy (MBE) deposition on single crystal substrates. For example, in Dingle et al., U.S. Pat. No. 4,261,771, the fabrication of monolayer semiconductors by one MBE technique is described. These layered or modulated prior art structures are typically called "superlattices." Superlattices are developed on the concept of layers of materials forming a one-dimensional periodic potential having a periodic length larger than a typical interatomic distance. The superlattices are formed by a periodic variation of composition or of impurity density. Typically, the largest period in these superlattices is on the order of a few hundred Angstroms; however, monatomic layered structures have also been constructed. One-dimensional superlattices can be characterized by the format of a layer of a substance A (such as GaAs) followed by a layer of a substance B (such as GaAlAs), in a repetitive manner; formed on a single crystal substrate. The desired superlattice is a single crystal synthetic material with good crystalline quality and long range order. The conventional superlattice concepts have been utilized for special electronic and optical effects.
In addition to superlattices, Dingle discloses quasi-superlattices and non-superlattice structures. The former are comprised of epitaxially grown islands of a foreign material in an otherwise homogeneous layered background material. Non-superlattice structures are an extension of quasi-superlattice materials in that the islands are grown into columns extending vertically through the homogeneous layered background material. These superlattice type structures suffer from the same problems that plague homogeneous crystalline materials. There is very little variation possible in the range of constituents and in the type of superlattices constructed, because of the requirement that the crystalline periodicity of layer substance A be approximately the same as that of the layer substance B at each interface of A and B. These superlattices are restricted to a small number of crystalline materials and the growth rates are constrained by the MBE technique.
In addition to the MBE type of superlattice construction techniques, other researchers have developed layered synthetic microstructures utilizing different forms of vapor deposition, including diode and magnetron sputtering and standard multisource evaporation. The layer dimensions are controlled by shutters or by moving the substrates relative to the material sources or controlling reactive gas partial pressure or with combinations of shutters and relative motion. The layers typically are formed in a structure in only one type of deposition system. The materials reported have been formed from crystalline layers, noncrystalline layers and mixtures thereof; however, each of the efforts so far reported is directed at the synthesis of superlattice-type structures by precisely reproducing the deposition conditions on a periodically reoccurring basis. These materials can be thought of as synthetic crystals or crystal analogues in which it is crucial that the long range periodicity, repetition of a particular combination of layers or grading of layer spacing be maintained. These structures are both structurally and chemically homogeneous in the x-y plane, but are periodic in the third (z) direction.
Other works which are assigned to the present assignee, as described for example in U.S. patent application Ser. No. 422,155, entitled Compositionally Varied Materials And Method For Synthesizing The Materials, describe forming various types of matrices and dispersing nonequilibrium configurations therein. The principles of layering and compositional modulation in the prior work is dependent upon the amounts of material distributed on an atomic and molecular scale throughout the material. For example, as described in some of the prior work, very small amounts of material were added to a matrix to increase the bulk resistance properties of the resulting material. The addition of larger amounts of material to the matrix decreases the bulk resistance properties. The principles are utilized to modify a range of materials from dielectric to semiconducting to metallic materials. In metallic materials, the addition of larger amounts of material to the matrix does not necessarily decrease the bulk resistance properties. The superconducting properties of the materials also can be enhanced. These techniques provide a distribution of material configurations on an atomic and molecular scale from microscopic to macroscopic configurations in the matrix material. The distribution of compositional changes of a primarily nonequilibrium nature lead from individual atoms or groups of atoms to layering when sufficient amounts of material are introduced into the matrix. These techniques provide significant control of material properties such as thermal and electrical conductivity and other parameters by introducing atoms and alloys into materials which bond in a manner not previously described.